African trypanosomiasis is once again a major public health problem throughout much of Africa. Control strategies are limited due to the lack of mammalian vaccines and efficacious and affordable drugs. The recent developments in recombinant DNA technologies stand to improve existing vector control measures, but progress in this are has been hampered by the lack of information on the molecular aspects of tsetse and its interactions with trypanosomes. Tsetse flies are in general refractory to the transmission of African trypanosomes although the molecular basis for refractoriness is unknown. Prior induction of the tsetse immune system can reduce parasite transmission indicating that tsetse's innate defense surveillance system is capable of terminating the development of trypanosomes. In the work proposed here, a comprehensive panel of immune-responsive genes will be identified using several different molecular strategies and their expression profiles in response to different immune challenges will be analyzed. Both systemic and local responses, respectively mediated by fat body and proventriculus tissue, will be investigated at the molecular level. Functional studies are proposed with several key antimicrobial gene products to understand their role in parasite transmission. This study will allow us to begin to understand the components of the tsetse immune system and the interactions they exhibit with trypanosomes during the parasite establishment and transmission in the fly. Although germ line transformation approach is difficult in tsetse because of its unusual reproductive biology, expression of transgenes in the gut symbionts of tsetse, particularly Sodalis glossinidius, may prove a valuable strategy to control disease transmission. Central to such a strategy will be identification of suitable genes for use as transgenes. Immunity genes are strong candidates, which this study may help to identify. These genes may also play a role in differentiation of trypanosomes and again, this project will identify potential candidates.